The manufacture of fiber optic cable involves drawing a preform to form a fiber. The drawn fiber is coated with a photocurable coating, and then exposed to ultraviolet (UV) radiation to cure the coating. The UV light activates photoinitiators in the coating that trigger the coating reaction as a whole. Typically, an irradiator tube, which is transparent to UV radiation, defines a space through which the fiber travels. The irradiator tube maintains an atmosphere which is inert with respect to the coating on the fiber, and also shields the fiber from vigorously circulated lamp coolant (blown air for example).
The polymerization (or curing) reaction is affected by several factors. First, oxygen in the irradiator tube inhibits the polymerization reaction. Oxygen may exist in the coating starting materials and in the irradiator tube during the cure process. The oxygen very rapidly combines with the activated photoinitiators in the coating to form a chemical with low reactivity. This essentially halts the curing reaction. Conventionally, therefore, an inert gas flow is provided through the irradiator tube to displace the oxygen in the fiber curing environment.
Second, the polymerization reaction rate is proportional to the UV light intensity to which the coating is exposed. Conventionally, therefore, by-products (or fumes) of the polymerization reaction are exhausted from the irradiator tube. Otherwise, these by-products would deposit on the irradiator tube and reduce the UV radiation impinging on the coating.
Third, it is important to strike a balance between the low oxygen concentration within the irradiator tube and the exhaustion of the coating cure by-products (or fumes) from the irradiator tube. For a particular application, the flow rate of the inert gas remains constant, and the flow rate of the exhaust is adjusted to achieve the desired balance. For example, if the exhaust flow is increased, the coating cure fumes are more effectively exhausted out of the irradiator tube. However, if the exhaust flow is too high, too much oxygen enters into the irradiator tube, thereby defeating the optimization of the curing reaction.
The conventional techniques for curing the coating on a fiber involve providing an inert gas flow through the irradiator tube and exhausting the curing fumes from the irradiator tube. Although these conventional techniques are generally thought to be acceptable, they are not without shortcomings. In particular, excessive amounts of oxygen and curing fumes still remain in the irradiator tube, thereby reducing the curing reaction efficiency. The curing fumes are particularly problematic because they darken the irradiator tube based on curing throughput. Thus, the irradiator tube must be periodically replaced. The replacement process reduces manufacture yields and limits the size of the draw run.
Further, the inert gas flows within the irradiator tube cause the fiber to vibrate when drawn. These vibrations may be inadvertently detected by a device that monitors the quality of the coating. As a result, the quality inspecting device may incorrectly determine that the fiber coating has defects.